"I think we definitely want to meet the Mirror Master next year," said Kreisberg. "I'm sure Dr. Alchemy will show up at some point. And we've got one thing that hopefully people will lose their minds over."

Mirror Master, a villain equipped with mirrors capable of projecting holograms and hypnotizing opponents, first appeared back in 1959's "Flash" #105. A version of the character, played by ex-"Partridge Family" heartthrob David Cassidy, appeared in the 1990 "Flash" series. Dr. Alchemy's comic book history stretches further back to 1958; the character uses a Philosopher's Stone to transmute elements and has been a member of the Rogues in the past.

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There's one villain that we may have already met, albeit in their civilian identity. Danielle Panabaker's Caitlin Snow has an alter-ego in the comics -- the Firestorm villain Killer Frost. The actress said she's "really excited" about her character potentially following the source material and becoming Killer Frost. "You're going to see her sooner than you think," Panabaker said. It's worth noting that the Snow version of Killer Frost, introduced just before "Flash's" premiere in 2013, is a villain of Firestorm; on the CW series, Snow's love interest is Firestorm, a character played by Robbie Amell.